Lecture 10 Group 2A Chemistry, Lewis Diagrams

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Lecture 10 - Group 2A Chemistry, Lewis Diagrams
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Review

• Lewis dot diagrams
• Formulas of ionic compounds
• Electronegativity
• Ionic, covalent, polar covalent bonds

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Types of Bonds
Polar Covalent Bond
Covalent
Ionic Bond
3.5
2.0
0
0.5
Electronegativity Difference
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Group 2A Chemistry

• Be, Mg, Ca, Sr, Ba, Ra
• all soft, low density metals
• M2 ions (ns2 valence electrons)
• form ionic compounds

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Alkaline Earth Metals

• MgO H2O Mg(OH)2(aq)
• Mg(OH)2(aq) Mg(aq)2 2 OH -(aq)

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Flame Tests
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Reactions of group 2A elements

• Mg X2 MgX2 X F, Cl, Br, I
• 2 Mg O2 2 MgO
• 3 Mg N2 Mg3N2

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Producing Mg

• The Dow Process
• CaCO3(s) CaO(s) CO2(g)
• CaO H2O Ca(OH)2 OH-(aq)
• Mg2(aq) 2 OH-(aq) Mg(OH)2(s)
• Mg(OH)2(s) 2 HCl(aq) MgCl2(aq) 2 H2O(l)
• MgCl2(s) Mg(l) Cl2(g) (by electrolysis)

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Uses of Mg

• Metal Density (g/mL)
• Fe 7.9
• Al 2.7
• Cu 8.9
• Ni 8.9
• Mg 1.7 (high strength/weight)

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Other Uses of Mg

• TiCl4 2 Mg Ti 2 MgCl2
• BeF2 Mg Be MgF2
• UF4 2 Mg U 2 MgF2

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Calcium

• slaked lime sand water air ?
• Ca(OH)2(aq) CO2(g) CaCO3(s) H2O(l)
• cement

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Calcium and Groundwater

• CO2(g) H2O(l) ? H2CO3(aq)
• CaCO3(s) H2CO3(aq) ? Ca(HCO3)2(aq)

carbonic acid
limestone
soluble!
? stalactites, stalagmites
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Hard Water

• water containing lots of Ca2, Mg2
• Problem 1 Scale
• (Ca2, Mg2)(aq) CO2(g) H2O ? CaCO3(s)
  MgCO3(s)

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Hard Water

• water containing lots of Ca2, Mg2
• Problem 2 Soaps
• 2 CH3(CH2)14COO-Na Ca2

dissolves in water
dissolves in dirt
Ca(CH3(CH2)14COO)2(s) 2 Na(aq)
(insoluble soap scum)
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Softening Water

• by ion exchange
• Na2R(s) Ca2(aq) ? CaR(s) 2 Na(aq)

An ion exchange resin
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Lewis Diagrams for Covalent Compounds

• predict where the electrons are in a molecule

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Lewis Diagrams for Covalent Compounds

• predict where the electrons are in a molecule
• needed to predict the shape of a molecule

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Lewis Diagrams for Covalent Compounds

• e.g. NF3
• Step 1. Count the valence electrons
• 5 (3 x 7) 26

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Lewis Diagrams for Covalent Compounds

• Step 2. Write a skeletal structure. Use the least
  electronegative atom in the centre

F
F
N
a pair of e- (a single bond)
F
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Lewis Diagrams for Covalent Compounds

• Step 3. Complete the octets for each terminal
  atom (except H)

..
..


F
F
N
..
..


F
..
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Lewis Diagrams for Covalent Compounds

• Step 4. Assign any additional electrons as lone
  pairs on the central atom

..
..
..


F
F
N
..
..


F
..
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Lewis Diagrams for Covalent Compounds

• Example 2. COCl2 (24 electrons)

..
..


C
Cl
Cl
..
..


O
..
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Lewis Diagrams for Covalent Compounds

• Rule 5. Make multiple bonds where necessary to
  complete the octets

..
..


C
Cl
Cl
..
..


O
..
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Lewis Diagrams for Covalent Compounds

• Rule 5. Make multiple bonds where necessary to
  complete the octets

..
..


C
Cl
Cl
..
..

O
..
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Lewis Diagrams for Covalent Compounds

• Example 3. Chlorate ion, ClO3-
• ((1 x 7) (3 x 6) 1) 26

-
..
..
..


Cl
O
O
..
..


O
..
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Sometimes its not so clear...

• e.g. CS2
• (both electronegativities 2.5)
• is it SCS or CSS ?

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Formal Charge

• the number of valence electrons in the free
  neutral atom
• -
• the number assigned to the atom in the Lewis
  structure

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..
..


CSS

• C S S
• Valence electrons 4 6 6
• Electrons assigned 6 4 6
• Formal Charge -2 2 0

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..
..


SCS

• S C S
• Valence electrons 6 4 6
• Electrons assigned 6 4 6
• Formal Charge 0 0 0